As high-speed metal cutting at a feed per one edge exceeding 0.3 mm becomes prevailing, conventional hard-coated tools have become insufficient in oxidation resistance, wear resistance, seizure resistance, peel resistance, etc. of hard coatings. Accordingly, various technologies have been proposed to improve oxidation resistance, wear resistance, seizure resistance, peel resistance, etc. of the hard coatings.
JP 2003-225807 A discloses a cutting tool having a hard coating layer exhibiting excellent wear resistance in high-speed cutting, the hard coating layer being formed by physically depositing a composite nitride of Ti and Y to an average thickness of 1-15 μm on a substrate of tungsten carbide-based cemented carbide or titanium carbonitride cermet, the hard coating layer having maximum-Y-component points (minimum-Ti-component points) and Y-component-free points (TiN points) alternately at a predetermined interval in a layer thickness direction, the concentration of the Y component changing smoothly between the maximum-Y-component point and the Y-component-free point, the maximum-Y-component points having a composition represented by the formula of (Ti1-xYx)N, wherein x is 0.05-0.15 by atomic ratio, an interval between the adjacent maximum-Y-component point and the Y-component-free point being 0.01-0.1 μm.
Japanese Patent 3,460,288 discloses a wear-resistant coated member comprising a substrate and a hard coating formed on its surface, the hard coating being formed by layers of nitrides, oxides, carbides, carbonitrides or borides of 2 or more elements selected from the group consisting of metal elements of Groups 4a, 5a and 6a, Al and Si, such that their compositions change continuously at a period of 0.4 nm to 50 nm to a total thickness of 0.5-10 μm.
Both hard coatings of JP 2003-225807 A and Japanese Patent 3,460,288 are multi-layer coatings having repeatedly changing concentrations or compositions. However, because any of the above technologies uses only an arc-discharge ion plating method, they do not necessarily have satisfactory seizure resistance in the cutting of steel, etc., on which seizure tends to occur.
JP 64-83656 A discloses a method for producing a hard coating by an electron beam method using a heated filament and a magnetron sputtering method in a vacuum chamber. However, because the electron beam method generates a low plasma density, the resultant hard coating has much lower adhesion than those obtained by an arc-discharge ion-plating method. Accordingly, hard coatings obtained by this method do not have sufficient hardness, strength and toughness.
JP 8-170167 A discloses on a wear-resistant hard coating formed on a substrate surface, comprising a first layer on the substrate side having a chemical composition represented by (AlxTi1-x-ySiy)(CzN1-z), wherein 0.05≦x≦0.75, 0.01≦y≦0.1, and 0≦z≦0.4, and a second layer on the surface side comprising BN.
JP 2000-326108 A discloses a tool having a hard coating formed on a substrate made of high-speed steel, cemented carbide, cermets or ceramics, the hard coating comprising a C layer made of a nitride of a Ti-based metal and having a thickness of 0.1-1 μm, an A layer made of a nitride, carbonitride, oxinitride or oxicarbonitride of a metal comprising 10-60 atomic % of Si, and less than 10 atomic % of at least one selected from the group consisting of B, Al, V, Cr, Y, Zr, Nb, Mo, Hf, Ta and W, the balance being Ti, based on the metal only, which comprises Si3N4 and Si as independent phases, and a B layer made of a nitride, carbonitride, oxinitride or oxicarbonitride of a metal comprising more than 40 atomic % and 75 atomic % or less of Al, and less than 10 atomic % of at least one selected from the group consisting of B, Si, V, Cr, Y, Zr, Nb, Mo, Hf, Ta and W, the balance being Ti, based on the metal only, one or more A layers and one or more B layers being alternately formed, and the B layer being formed immediately on the C layer.
In JP 8-170167 A and JP 2000-326108 A, however, only an arc-discharge ion-plating method is carried out, thereby providing insufficient seizure resistance in the cutting of steel vulnerable to seizure in its edge, etc.
JP 8-127863 A discloses a wear-resistant, hard laminate coating comprising as main components at least one element selected from the group consisting of elements of Groups IVa, Va and VIa in the Periodic Table, Al, Si and B, and at least one element selected from the group consisting of B, C, N and O, the coating having at least 2 types of compound layers having different compositions and composition-changing layers, whose element compositions change in a thickness direction between the compound layers, the compound layers and the composition-changing layers being periodically laminated, and a crystal lattice being continuous over one period or more between the layers.
JP 11-509580 A discloses a method for forming a high-lubrication, hard coating comprising, for instance, molybdenum disulfide and TiN on a cutting tool, using a sputtering-ion plating system comprising a first target made of a metal sulfide (for instance, molybdenum disulfide), and a second target made of at least one metal selected from the group consisting of titanium, vanadium, chromium, zirconium, niobium, molybdenum, tantalum, hafnium and tungsten.
However, JP 8-127863 A and JP 11-509580 A do not provide hard coatings with sufficient adhesion and hardness, failing to sufficiently improve the wear resistance of cutting tools.
Japanese Patent 3,416,938 discloses a laminated hard coating comprising a compound layer mainly composed of at least one first element selected from the group consisting of metal elements in Groups IVa, Va and VIa in the Periodic Table, Al, Si and B, and at least one second element selected from the group consisting of C, N and O, and a composition-changing layer of TiC, BC, WC, TiCN, TiN, TiB, TiBN, BN, SiN, etc., the compound layers and the composition-changing layers being laminated such that a composition periodically changes in a thickness direction, and a crystal lattice being continuous with deformation in the composition-changing layer.
JP 2001-293601 A discloses a cutting tool having a wear-resistant hard coating formed on a tool substrate, the coating comprising as a main component a nitride or carbonitride of at least one element selected from the group consisting of elements of Groups 4a, 5a and 6a in the Periodic Table and Al, the substrate being made of at least one selected from the group consisting of WC-based cemented carbide, cermet, silicon carbide, silicon nitride, aluminum nitride, alumina, boron carbide, sintered aluminum oxide-titanium carbide, high-speed steel, die steel and stainless steel, the wear-resistant, hard coating containing at least one type of fine, hard particles selected from the group consisting of B4C, BN, TiB2, TiB, TiC, WC, SiC, SiNx (x=0.5-1.33) and Al2O3.
However, any of the hard coatings described in Japanese Patent 3,416,938 and JP 2001-293601 A fails to meet the demand of having sufficient lubrication capable of withstanding dry cutting conditions while maintaining oxidation resistance and wear resistance.